Immunity and its types
Immunity can be defined as the self-preparedness (of the body) against invasion by microbes (viruses, bacteria, fungi, parasites) which also include defence against non-microbial antigens and malignancy. Classically, it is divided into two categories: the non-specific and the specific immunity.
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The non-specific immunity or the innate immunity is not affected
by prior contact with the invader and it is effective against all without
recognising the specific identities of the enemies. It is composed of many
factors. Firstly the constitutional factor due to which some individuals are
more prone to some specific infections but others are not. Next comes the
general barrier against infections which is provided by the intact skin and the
mucous membrane. The skin is supported by the protective roles of sebum, sweat,
etc., along with the commensals, the bacteria normally present in the skin. The
mucous membrane is further protected by the layer of mucous and the
antimicrobial chemicals on it. The acid in stomach kills the microbes entering
with food and the intestinal bacterial flora (commensals) prevents growth of
pathogens (microbes causing disease). The ciliary action in the respiratory
passage also provides important defence.
The phagocytes, i.e., the neutrophils and the macrophages form a very important part of the non-specific immunity. They provide resistance against almost all types of microbes by engulfing them and also by producing different toxic materials to kill the microbes. But they act better, as we shall see, in association with the specific immune mechanism. The phagocytes also play an important role in inflammation. Inflammation is a protective reaction not only against microbes but also against any injury. Its aim is to inactivate the offending agents. The complement and the properdin system (see later) similarly can also be included in the innate immunity. The eosinophils do their best in parasitic infection. If any of the above is damaged or deranged, infections occur very easily. Innate immunity is thus very important in protecting the individual.
The specific immunity also called acquired immunity is primarily
the function of the lymphocytes and is helped by other cells also. In this type
there is separate mechanism for each and every enemy (antigen). The weapon for
one invader will not act against the other, hence called specific. This
immunity develops properly, only when the body is exposed to the specific
antigen, i.e., prior contact is essential, but the provision is inborn.
Specific immunity develops only against antigens which are not recognised as
self, i.e., foreign (= non- self) to the immune system.
Antigens are substances which evoke an immune response when introduced in the body. Antibodies are proteins,
produced against antigens within the body by the plasma cells.Foreign proteins behave as powerful antigens. Antigens are also formed by combination of proteins with other substances, e.g, polysaccharides, lipids, nucleic acids and even simple molecules like haptens.
Hapten is that portion of an antigen which can determine its immunologic specificity but is not self sufficient to produce immune response. It can be a very small molecule like DNP (dinitrophenol). Hapten can combine with the specific antibodies, but, it alone cannot lead to production of antibodies. When the haptens are combined with aprotein, the resultant molecule becomes a complete antigen and can stimulate antibody production. Haptens are obtained from drugs, products of animal skin and scales, chemicals, particles of dust, etc.
The antigens of virus, bacteria, fungi and others, enter into the body as a part of the invading organism through the food or air we take along with other innumerable antigens. These can also enter through the injured skin or mucous membrane.
The new proteins produced by the cancer cells, the virus infected cells and the proteins in the transplanted tissues are also treated as foreign antigens, as these differ from the body proteins immunologically. In abnormal situations some substances of the body also behave as antigen (auto-immune disease).
The specific immunity may be active or passive. Passive immunity
occurs due to transfer or introduction of antibodies (immunoglobulins) from
outside, e.g., injection of serum containing specific antibodies. Some examples
are anti-tetanus serum (ATS) containing antibodies against bacteria causing
tetanus, similarly there are anti-gasgangrene serum (AGS), antivenom serum
(AVS), etc. Different monoclonal antibodies are also used now a days (see
later). In the natural course, passive immunity occurs due to transfer of
antibodies from mother to the foetus through placenta or through the breast
milk to the baby. Passive immunity is a ready made immunity and can act
immediately but it is short lasting.
Active immunity is developed within the body by the lymphocytes against antigens introduced in to the body. In the natural course, active immunity develops after infection by a microbe or introduction of an antigen by any means into the body. The respective lymphocytes are then activated. Active immunity can also be developed artificially by vaccination, e.g.,, polio vaccine. Similarly, there are triple antigens, tetanus vaccine (tetanus toxoid), etc. (Harmless antigenic components of the microbes are used in vaccines). Active immunity once developed in the body is remembered by the concerned cells for a long time It is reinforced at times by a booster dose of the vaccine. Unlike the passive immunity, active immunity requires some time to operate. The active immunity which is produced within the body is again of two types depending on the nature of the invaders (antigens). These are cell mediated immunity (CMI) and humoral immunity. In humoral immunity, antibodies are produced, but in cell-mediated-emmunity, the lymphocytes themselves act.